Stress vs Strain – Which Comes First and Why?

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It’s Stress vs Strain, and you won’t want to miss the action!

‘Stress‘ and ‘Strain‘ are the most encountered terms when it comes to studying Elasticity. Though it might not have such a huge weightage in competitive exams like JEE/NEET, but trust me, ladies and gentlemen: “There’s no Mechanical Engineering without these 2 terms” – Being a student pursuing my UG degree in Mechanical, I can say this with full assurance. And if there’s no Mechanical, there are no cars, bridges, buildings, etc.

Through this article, we are going to find out which one of the two comes first – It’s going to be Stress vs Strain?

Table Of Contents
  1. 1. Introduction to Stress:
  2. 2. Introduction to Strain:
  3. 3. Final Decision: Stress vs Strain | Who wins the Match?

1. Introduction to Stress:

Stress is defined as the Internal Restoring Force acting per unit area.

Now, What is this Internal Restoring Force? Let’s understand the process to know what happens inside the material.

The atoms inside the solid are arranged in a spring-ball system. So, when a load (external force) is applied, it disturbs the equilibrium state by deforming the springs. This deformation is responsible for the Internal restoring force, and we call it restoring because it tends to bring the system back to its equilibrium.

Fig. Spring-ball arrangement in Solids

The following flow-chart explains the process :

Explanation of Flowchart (Expand This)

First, the external force is applied. As we have already discussed, the spring ball system is an analogy for the arrangement of atoms in bodies. This external force causes a disturbance in this system. As we all know, springs are elastic in nature, and hence, they give birth to a restoring force in order to restore back to equilibrium. This indirectly induces stress in the body, as stress is defined as Restoring Force per unit area

The flowchart shows the insider view of what happens inside the body when an external force is applied. This actually decides who's the winner in Stress vs Strain
Fig. Flowchart

2. Introduction to Strain:

Strain is defined as ‘Change in Dimensions / Original Dimensions.’

Strain=ChangeOriginalStrain = \frac{Change}{Original}

Again, there are types of strain :

  • Longitudinal Strain – Change happens in the length
  • Shear Strain – There is a shift that leads to an angle change
  • Volumetric Strain – Change happens in the Volume
Representation of what happens during longitudinal strain
Fig. Longitudinal Strain (expansion)
Representation of what happens during shear strain
Fig. Shear Strain (measured as angle)
Representation of what happens during volumetric strain
Fig. Volumetric Strain (compression)

Important Note :
A careful observation of the Flowchart above would tell that: There is ‘Strain’ coming into the picture at the second step, since on applying load, there is a deformation happening. This is exactly what we discuss in Strain.

The flowchart now reveals the secret and decides who's the actual winner in the Stress vs Strain match
Explanation of the Flowchart (Expand This)

Here, we are actually revealing the secret – The disturbance that the external force caused inside the atom arrangement was nothing but the strain that came into existence, and then you get the restoring force.

3. Final Decision: Stress vs Strain | Who wins the Match?

Now, it’s very much clear from the above discussion that: it’s the Strain which comes first! All of this is because of the Definition of Stress. Most of the time, we just memorize the formula of Stress as ‘Force/Area’, which is not complete.

  • The Complete Answer is: Stress = Restoring Force/Area
Strain wins the Stress vs Strain match

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